Because of their improved bandwidth properties, optical signals are being increasing utilized for the transmission of data. Further, the bandwidth capacity of a given fiber optic cable can be further increased by transporting a multiplicity of independent signals within a single fiber on separate channels at slightly different wavelengths, a technique known as wavelength division multiplexing (WDM). Thus, for example, a nominally 1550 nm fiber optics signal might comprise four or eight channels or even 64 or more channels, each separated by 0.8 nm (corresponding to 100 GHz) or 1.6 nm (corresponding to 200 GHz). However, for such signals to be useful, it must be possible to switch the optical signals coming in on an optical fiber (or other optical conduit) to a fiber/conduit leading to a desired destination. More generally, in complex fiber optic structures such as those used in the telecommunications industry and for sensor and computer data networks, light signals must be efficiently routed or switched from an array of N incoming optical fibers, which fibers may be single mode or multimode, to an array of M outgoing optical fibers. Such a switch will sometime be referred to hereinafter as an N.times.M switch or crossconnect.
While a number of techniques have been proposed over the years for performing N.times.M switching optically, none of these techniques have proved to meet all requirements simultaneously. This is partly due to the varied architectures which are required for such switches. For example, an N fiber in, N Fiber out (N.times.N) switch that maps each incoming fiber optical signal to one and only one fiber output is termed an N.times.N crossconnect. It is nonblocking if any connection is possible, without regard to earlier established connections. For some applications, reconfigurably nonblocking switches are sufficient. In other applications, switches that multicast or broadcast, sending one incoming signal to more than one output, or that perform other variant functions, are required. The data capacity demands on fiber optic networks are also becoming more complex, imposing a requirement that switching technologies be scalable so as to be extendable in a straight forth manner from small switches (for example 2.times.2 or 4.times.4 to larger switches such as 64.times.64, 1024.times.1024, and beyond). It is also desirable that such switches be integrable such that individual miniaturized switching elements can be combined with many others on a single chip or substrate to provide a larger N.times.N or N.times.M crossconnect structure. However, designing such structures, particularly for larger switches, is very complex even for single channel operation, and the complexity increases dramatically for multichannel WDM operation. Multichannel operation facilitates an increasingly important advantage of optically transparent transport and switching which is that several noninteracting signals may pass through the switch simultaneously which signals convey entirely incompatible data rates, encodings and protocols in parallel without compromising one another.
Another requirement for optical switches of the type described above in particular, and for optical components and structures in general, is that they efficiently interface with optical fibers. This is true because, while optical signals can travel through free space, to minimize signal losses and to optimize flexibility in transmission, most optical signals used in telecommunications are transmitted through optical fibers. Other key performance parameters include minimizing insertion loss, crosstalk and polarization sensitivity. Low operating power and high reliability are also important.
Because satisfactory products for performing such optical switching have not existed, it has therefor been necessary to convert optical signals to be switched into electrical signal for switching and to then reconvert the signals to optical signals for outputting. This technique can be expensive, time consuming, impose bandwidth limitations on the system and introduce several sources of potential error. It can also limit the flexibility of the system and is generally not an efficient way to operate.
In addition to the switching applications discussed above, there are numerous applications where a need exists to be able to change the direction in which an optical signal is passing through a waveguide, filter an optical signal, particularly a multiwavelength or multichannel signal so as to selectively pass or block various of the multiwavelengths or multichannels, and/or to selectively couple the multiwavelength or multichannel along optical paths out of the plane of the waveguide.. There are also applications where a need exists to perform more complex transformations on an optical signal. For example, it may be desired to alter the propagation from a guided mode characteristic of the input optical signal in the optical component or structure before outputting it to a radiated or free space mode (or other unguided mode) or it may be desirable to have an optical component which can selectively (i.e. switchable) focus an incoming beam in a desired direction, or perform other complex optical functions on the input. It should be possible to perform all of these functions utilizing optical components and/or structures which are relatively easy and inexpensive to fabricate.